Architecture, design, and test of continuous-time tunable intermediate-frequency bandpass delta-sigma modulators

摘要

This paper examines the architecture, design, and test of continuous-time tunable intermediate-frequency (IF) fourth-order bandpass delta-sigma (BP△Σ) modulators. Bandpass modulators sampling at high IFs (～200 MHz) allow direct sampling of the RF signal-reducing analog hardware and make it easier to realize completely software programmable receivers. This paper presents circuit design of and test results from continuous-time fourth-order BP △Σ modulators fabricated in Al/As/GaInAs heterojunction bipolar technology with a peak unity current gain cutoff frequency (f{sub}T) of 80 GHz and a maximum frequency of oscillation (f{sub}(MAX)) of about 130 GHz. Operating from±5-V power supplies, a fabricated 180-MHz IF fourth-order △Σ modulator sampling at 4 GS/s demonstrates stable behavior and achieves 75.8 dB of signal-to-(noise + distortion)-ratio (SNDR) over a 1-MHz bandwidth. Narrowband performance (～1 MHz) performance of these modulators is limited by thermal/device noise while broadband performance (～60 MHz) is limited by quantization noise. The high sampling frequency (4 GS/s) in this converter is dictated by broadband (60 MHz) performance requirements.